Cable television has become a staple product in many homes. An estimated 60-65% of all American households now receive cable service. Cable companies install extensive cable networks to provide cable service. A typical cable network includes four main elements: a headend, a trunk system, a distribution system, and subscriber drops. The headend receives cable programming from many sources, including satellite, over-the-air local station signals, and terrestrial microwave links. The headend processes the received programming and delivers it over the trunk system, which is the main transmission artery of the cable network. The transmission lines of a modem trunk system are typically a combination of coaxial cable and fiber optic cable. The trunk system branches into a number of distribution systems. The transmission lines of the distribution system are also generally a combination of coaxial cable and fiber optic cable. The distribution system delivers the programming from the trunk system into individual subscriber areas. The distribution system is also called the "feeder."
The distribution system terminates in the subscriber area at a distribution point, such as an optical network unit. A subscriber drop completes the connection from the distribution point to the subscriber location, typically a house or apartment. Due to the current economic limitations of fiber optics, the subscriber drop transmission line from the distribution point to the subscriber location is generally coaxial cable.
The cable network generally ends at the junction between the subscriber drop and the subscriber location. The subscriber is responsible for finishing the connection within the subscriber location. Generally, coaxial cable is used as the transmission line to distribute the cable programming to television sets within the subscriber's location. Often, a set-top box is used within the subscriber location to decode signals for premium channels, pay-per-view or the like.
To provide cable service to cable subscribers, a cable company must install and maintain hundreds or thousands of miles of transmission lines. Both fiber optic cable and coaxial cable transmission lines are used in abundance in a typical cable network. Consequently, cable companies are constantly striving to create new methods to simplify and reduce the costs of the maintenance process. Much of the burden of maintaining the cable network involves ensuring a continuous transmission line from the headend to the subscriber location. For the purpose of this discussion, a continuous transmission line is one which provides an uninterrupted electrical path from one end of the transmission line to the distal end of the transmission line.
A typical maintenance problem takes the following form. A subscriber calls the cable company to announce a problem with the subscriber's service. Perhaps the subscriber thinks that he or she is not receiving a signal at all. Once the cable company receives the call, it must proceed to troubleshoot the problem. Usually, the first step to troubleshoot the problem is to ensure that the cable connection is continuous from the headend all the way to the subscriber location. The cable company typically has mechanisms in place to test the continuity of the transmission line from the headend to the distribution point. However, testing the continuity of the transmission line from the distribution point to the subscriber's location can be difficult. Often the testing requires sending a field technician to the subscriber's location to manually test the conductivity of the transmission line with continuity testers.
The telecommunications industry has addressed the similar problem of testing telecommunications transmission lines for continuity from a service provider to a subscriber location, and developed a workable method for testing the continuity remotely, usually without having to dispatch a field technician. In a telecommunications maintenance situation similar to that above, a service provider can electrically test a unique telecommunications transmission line. If the transmission line contains a discontinuity, such as an open or a short, the discontinuity is detected at a testing location. By using measurement devices at the testing location, the service provider can measure the characteristics of a reflected signal, and, based on those characteristics, can pin-point a discontinuity practically anywhere along the transmission line to the subscriber's location. However, there are important distinctions between a telecommunications network and a cable network which make a similar testing scheme unworkable if applied to a cable network.
Each subscriber in a telecommunications network is individually addressable. For the purposes of this discussion, the term "addressability" means the ability for a service provider to identify and transmit a signal to a particular subscriber in the network without transmitting that signal to any other subscriber in the network. Accordingly, the service provider can establish a circuit from the testing location directly to a particular subscriber, in similar fashion to establishing a telephone call between two subscribers. The established circuit spans only the transmission lines between the testing location and the particular subscriber, and signals communicated over the circuit travel along only those transmission lines. In this manner, the tested transmission path applies to the particular subscriber, but not other subscribers in the telecommunications network. Consequently, a reflected signal on that circuit can only result from a discontinuity in the transmission lines supporting the circuit between the testing location and that particular subscriber.
In contrast, a cable network is distributive and generally nonaddressable. For the purpose of this discussion, the term "non-addressability" means the inability to identify and transmit a signal to a particular subscriber in the network without transmitting that signal to more than one subscriber in the network. In the cable network, a common set of programming signals is distributed from the headend to every subscriber. The cable network topology in use by most cable companies today does not include a manageable way to transmit a tone along a particular transmission line to a particular subscriber. Attempting to send a tone to only a particular subscriber is difficult if not impossible because the subscribers are not individually addressable. Sending a test signal into a transmission line in the cable network simultaneously transmits the signal to all of the subscribers downstream from the distribution point (if the signal were generated at the distribution point). If a reflected signal occurred, the cable company could not identify on which transmission line the signal was reflected. Consequently, the cable industry cannot use the testing scheme of the telecommunications industry to test the continuity of a transmission line from the cable company headend to the termination of the cable network at the subscriber location.
A method and system for remotely testing the continuity of a transmission line in a non-addressable network has eluded those skilled in the art. Accordingly, a need exists for a method and system to create the ability to remotely test a transmission line for continuity all the way through a non-addressable network, such as a cable network, to a subscriber location.